The present invention relates to the art of diagnostic imaging. It finds particular application in conjunction with single-photon emission computed tomography (SPECT) with multi-headed cameras and will be described with particular reference thereto. It is to be appreciated, however, that the invention will also find application in other non-invasive investigation techniques such as positron emission tomography (PET) and other diagnostic modes in which a subject is examined for emitted radiation.
Heretofore, single photon emission computed tomography has been used to study the radionuclide distribution in subjects. Typically, one or more radiopharmaceuticals were injected into a patient. The radiopharmaceuticals were commonly injected into the patient's blood stream for imaging the circulatory system or for imaging specific organs which absorb the injected radiopharmaceuticals. Gamma or scintillation camera heads were placed closely adjacent to a surface of the patient to monitor and record emitted radiation. In single photon-emission computed tomography, the head was rotated or indexed around the subject to monitor the emitted radiation from a plurality of directions. The monitored radiation data from the multiplicity of directions was reconstructed into a three dimensional image representation of the radiopharmaceutical distribution within the patient.
One of the problems with the SPECT imaging technique is that photon absorption and scatter by portions of the subject between the emitting radionuclide and the camera head distorted the resultant image. One solution for compensating for photon attenuation was to assume uniform photon attenuation throughout the subject. That is, the patient was assumed to be completely homogenous in terms of radiation attenuation with no distinction made for bone, soft tissue, lung, etc. This enabled attenuation estimates to be made based on the surface contour of the subject. Of course, human subjects do not cause uniform radiation attenuation, especially in the chest.
In order to obtain more accurate radiation attenuation measurements, a direct measurement was made using transmission computed tomography techniques. That is, radiation was projected from a radiation source to the patient and radiation that was not attenuated was received by detectors at the opposite side. The source and detectors were rotated to collect data through a multiplicity of angles. This data was reconstructed into an image representation using conventional tomography algorithms. The radiation attenuation properties of the subject from the transmission computed tomography image were used to correct for radiation attenuation in a later SPECT or other emission study.
One of the problems with this two step technique resided in registering the transmission computed tomography and the SPECT or other emission study images. Any misalignment of the two images provided erroneous radiation attenuation information which impaired the diagnostic value of the reconstructed images. Registration was improved by using discrete extrinsic or intrinsic landmarks that were known to bear a constant relationship to the patient's anatomy during the two studies. Another technique was to use a three dimensional surface identification algorithm to construct numerical models of the external surface of the images. The numerical models were then translated, rotated, and descaled until an optimal match was found. Nonetheless, there was still significant uncertainty when combining images from different modalities. Moreover, inconvenience, cost, and double scan time were inevitable.
To overcome these disadvantages, simultaneous transmission and emission data acquisition was utilized. The gamma camera head was positioned on one surface of the subject and a large plane of a radiation source was disposed opposite the camera head, e.g. between the subject and a counterweight for the camera head. The patient was injected with a different radionuclide from the radionuclide in the large planar source. Using conventional dual radionuclide technology, the data from the injected or emitted radionuclide and the data from the larger planar source or transmitted radiation were separated. The transmitted data was reconstructed using parallel ray transmitted computed tomography algorithms to produce attenuation correction coefficients for use in the emitted radiation reconstruction.
One problem with using a large planar source resided in its large bulk and weight. The large size of the planar source prevented the use of systems with multiple gamma cameras. Another drawback was the poor counting statistics of parallel-beam geometry reconstructions. Stronger radiation sources could be utilized to compensate for the poor counting statistics, but the associated higher patient radiation exposures are undesirable.
The present invention contemplates a new and improved simultaneous transmission and emission tomography method and apparatus which overcomes the above-referenced problems and others.